@Mitch: I know it seems like you're fighting the tide with all the OED-quoters around here, but you are backed by "normal" (standard, modern, respected) dictionaries. See my answer.
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John YOct 9 '12 at 16:06

6 Answers
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You can use It is thundering, but the verb to describe the accompanying flashes is lighten, so you say It is thundering and lightening. This use of lighten is well attested in the Oxford English Dictionary, but it isn't all that common.

There's all sorts of words attested in the OED but people just don't use (nowadays). 'Lighten' for 'lightning' is one of them. Of the three entries for 'lightening', two nouns and an adjective, the adjective come close but not quite.
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MitchOct 9 '12 at 13:10

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@MattЭллен: if they do use it, I would see it as a spelling error.
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MitchOct 9 '12 at 13:28

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@PeterShor I've probably heard lightninging an order of magnitude more than lightening in everyday speech; and various strained phrasings from people who think lightninging sounds wrong, but don't know what to use, several times more often than either.
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Dan NeelyOct 9 '12 at 14:04

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@Mitch: No, I am not saying that. The OED has three citations for lightning as a verb. Two are in the past tense lightninged and one is in the present tense third person singular it lightnings and seems to be from a non-standard source. However, there are nine citations for lighten in this sense from, among others, the Authorised Version, Ben Jonson, Defoe, Scott, Byron and Housman. The first of Peter’s nGrams may be misleading in that it presumably includes nouns as well as verbs. In current usage, I suspect the verbal form is avoided wherever possible.
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Barrie EnglandOct 9 '12 at 14:55

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Here's how the situation seems to me: 1) "It is lightening", while supported by the OED, looks like either a spelling error or like the person means it's getting lighter outside. 2) "It is lightning" looks strange because "lightning" is a noun 99% of the time. I've never seen either this or (1) used. 3) "It is lightninging" is something I've heard people say periodically, but looks ludicrous in writing. It seems like our best option is to recommend "Lightning is flashing", "There is lightning", or some other paraphrase.
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alcasOct 9 '12 at 16:46

The answer is yes. It doesn't seem there is much argument that thundering is the correct verb form for "thunder happening right now". The question hinges on whether lightning is a valid verb for "lightning happening right now".

Note that lightninged is the past tense and past participle, lightning is the present participle, and lightnings is the third person singular present tense in the above entry (transcribed from the American Heritage Dictionary, Second College Edition). The linked M-W entry also explicitly lists lightninged and lightning as participles.

The issue here seems to be that many people don't realize that when the dictionary lists lightning in the inflected forms, it isn't just repeating the main entry for the hell of it. It's really an inflected form. So please, use thundering and lightning with confidence.

'lightening' cannot be correct for this. It would mean 'to lighten', 'to become lighter' as in less heavy.
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MitchOct 9 '12 at 12:14

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OED says otherwise, as shown in my answer. What are your grounds for thinking the OED is wrong?
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Roaring FishOct 9 '12 at 12:16

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Did you check your OED for 'lightening'? 1. The rendering light or lighter; alleviation (of pain, sorrow); †comforting, cheering.2. concr. Leaven. Obs. exc. dial. (see E.D.D.).a. The shedding or shining of light; suffusion with light, lighting up; fig. enlightenment, illumination. 'Lighten' does seem to be a synonym for 'to make lightning' but the OED fails to state that no one uses that word anymore, and it doesn't attest the derivation 'lightening' the way you think, the derivation 'lightening' even less so.
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MitchOct 9 '12 at 13:23

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Dictionaries don't list the continuous form of verbs as separate entries. The lemma is 'lighten', and they assume that we know how to put the affix '-ing' on the end to make a continuous form.
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Roaring FishOct 9 '12 at 13:41

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@Mitch - The first is highly confusing, given the other meanings of "lightening", but the second is ungrammatical. "There is lightning outside right now" is the way to go.
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Rex KerrOct 9 '12 at 15:47

"Thundering" is correct, but slightly ambiguous as to whether you mean one ongoing peal of thunder or repeated ongoing peals. "Lightning" is not correct (lightning is a noun, not a verb), and the technically correct version will lead to confusion ("lightening" may also mean that the cloud cover is growing lighter, letting in more light from the sun or moon, so the monsoon seems to be lessening). Rewrite using constructs like "continual thunder and lightning". You are unlikely to be clearly understood otherwise.

Consider using something based around the following constructs.
Adjust tense to suit. These largely deal with the fact that lightining is (usually) a noun by using the noun form of thunder rather than using "thundering" and then having to deal with a noun + verb mix.

We are continuing to experience intense lightning and thunder.

There is almost non-stop lightning and thunder.

The lightning and thunder is continuing / continues.

... continous lightning and thunder.

If you want to deal in a mix of noun and verb

There is loud is thundering accompanied by intense lightning.

It has been thundering with lightning for hours now.

There is ongoing loud thunder / thundering followed soon after by spectacular lightning.

"Drag along and beating" :-) - using a verb to "verbify" an accopnaying noun.

"Thunder & Lightning" is usually used as the term for the overall phenomen. Swapping that to "Lightning and thunder" would be less commomn but, again, would largely be unremarked. You can then add "ing" to the thunder effectively changing it from a noun to a verb and "carrying" lightning with it so it becomes a formn of collective verb. So

"All morning it has been Lightning & thundering"

This is 'cheating' but would be clearly understood. Some people would note in passing that you had bent the rules somewhat but few would be liable to remark. I'll say again that this is 'cheating", breaks several rules, and the construct arguably does not withstand close analysis . BUT if somebody used it in an eg newspaper story it would probably work.

Phone rings:
"Hello. John speaking.""John, it's Adam. I was wondering what the weather was like there?"
"Terrible! It's been lighting and thundering almost non stop all morning.!"

or eg "Terrible - its still lighting and thundering as much as it was when you left this morning".

'And now for something quite different' department -

Lightninging

Note that

the word "lightninging" is the "obvious verb",

dictionaries tend to eschew it,

about 11,000 Google hiits contain it and that

in an informal or humorous context it would pass muster and be understood.

Anyone using lightninging is doing so with conscious knowledge of being clever or quaint.